A number of types of implantable devices are in use to monitor and control the electrical activity of the heart. These devices include an implanted pacemaker and/or defibrillator connected to one or more transvenous leads having one or more electrodes. These devices can apply electrical pacing or defibrillation charges directly to the myocardial tissue of the heart. Generally, the electrodes can both sense the electrical activity of the heart and deliver an electrical stimulus provided by the pacemaker when required.
Combination pacing and defibrillation systems connected to a combination lead having both pacing and defibrillation electrodes implanted into the ventricle have also bean disclosed in the art. For these types of combination leads, the pacing electrode is located at the distal tip of the combination lead, with the defibrillation electrode generally spaced proximally from the distal tip yet sized to be positioned within the ventricle. The defibrillation electrode generally has a large surface area, as compared to the pacing electrode, because the defibrillation electrode delivers a substantially greater electrical stimulation charge over a greater mass of cardiac tissue.
It is preferable to have the large defibrillation charge delivered by the defibrillation electrode distributed over a substantial surface area of the heart, to avoid damaging the tissue by exposure to a high electrical charge over a limited surface area. However, the design of existing defibrillation electrodes of combination leads inherently provides an uneven charge distribution over its entire surface area, due to the electrical resistance of the electrode itself. As a result, damage to the heart tissue or to the blood may occur when a defibrillation pulse is applied. Moreover, inefficient distribution of the defibrillation pulse may result in unnecessarily high current having to be used to achieve cardioversion or defibrillation.
Accordingly, it would be beneficial to provide a lead for a cardioversion or defibrillation system which features an improved design for the defibrillation electrode capable of producing a more uniform charge distribution.